There are various diagnostic imaging systems such as X-ray, CT, ultrasonic wave, RI image, MRI, and so on. Since the MRI is not harmful to a human body as compared with other diagnostic imaging systems and provides images of the characteristics of structure material inside a human body, the MRI is a very important measuring apparatus in clinical practice.
The MRI apparatus may acquire data on parameters such as spin density, T1, T2, chemical shift, magnetization transfer, chemical exchange saturation transfer, blood stream and spectroscopy, which are unique information on the body, and may acquire various types of body images based on the parameter information.
EPI, which is a high-speed photographing technique capable of acquiring spatial frequency information required for image construction by one radio frequency (RF) pulse within a TR period, is one of the most rapid pulse sequences which are currently being commercialized, and is appropriate for images which are sensitive to a motion and require a fast temporal resolution, such as perfusion images, diffusion images, functional images, and the like.
However, since the EPI method fills the entire K-space within a TR period, there is geometric distortion in the image and therefore a distorted image may be easily obtained.
Further, the EPI method acquires data for one image within about 100 ms and therefore requires much more advanced hardware than that required for a general apparatus. That is, to sample data within the short period of time, there is a need to sufficiently widen and expand the receiving bandwidth, the maximum amplitude of a gradient magnetic field needs to be large, and a strong gradient magnetic field of 10 m T/m or more is generally required. Further, however the strong gradient magnetic field is applied, when a rise time up to the maximum amplitude is long, an image may not be obtained quickly, and therefore a gradient magnetic field apparatus having a slew rate of about 200 to 250 T/ms is required to increase the gradient magnetic field to the maximum amplitude within a short period of time. Further, the data is generated very rapidly in the EPI method, and therefore a high speed receiving apparatus capable of sampling at a rapid speed is required and a large amount of data needs to be processed within a short period of time, and therefore a sufficient amount of RAM is required.
However, the EPI image is deteriorated more than other pulse sequences in terms of a current resolution or contrast, and therefore is not used in a routine method.
Meanwhile, T2* relaxation refers to the decay of transverse magnetization caused by a combination of spin-spin relaxation magnetic field inhomogeneity.
A multi-echo gradient-echo pulse sequence, which is often used for acquisition of images to perform T2* mapping, needs a long scan time and commercialized sequences have limitations, including the number of echoes (a maximum of 12), memory capacity, and the dependency of the fitting functions.